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Internal receptors are located in the cytoplasm and respond to ligands that can cross the plasma membrane unassisted. What property must such a ligand have?
AIt must be hydrophilic
BIt must be a large protein
CIt must be hydrophobic
DIt must carry a negative charge
Answer & Solution
Correct answer: C. It must be hydrophobic
1. Internal receptors sit in the cytoplasm and bind ligands that can cross the plasma membrane on their own.
2. Only hydrophobic ligands can diffuse directly across the lipid interior of the plasma membrane.
3. Hydrophilic molecules cannot pass through the hydrophobic interior of the membrane, so they instead bind cell-surface receptors.
4. A large protein would also be unable to diffuse across the membrane, ruling that option out as well.
_Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 9 "Cell Communication", section 9.1 Signaling Molecules and Cellular Receptors_
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